Ceramic materials suitable for use as microwave filters for a specified application are selected based upon a number of physical and electrical properties of the ceramic materials. Among the most important properties for selecting a ceramic material for use in a specified microwave communication or radar device are: dielectric constant, K (also known as relative permittivity, ∈r), the quality factor, Q, the resonant frequency, f, and the temperature coefficient of resonant frequency, Tf.
The dielectric constant, K, of a material relates to the capacitance of the material (the ability to store electrical energy). The dielectric constant of a material, at least in part, determines the size of the filter necessary for a given application. Filter size is inversely related to the dielectric constant of the filter material. Relatively small filters can be fashioned from relatively high dielectric constant materials, whereas filter size must be increased as the dielectric constant is decreased. While very high dielectric constant materials may be considered desirable for miniaturization of equipment, practical considerations of design, as well as other physical properties of the ceramic filter material, such as the Q value and temperature coefficient of resonant frequency, will often dictate a choice of a relatively low dielectric constant material. Ceramic materials having a dielectric constant, K, in the range of about 30 to about 100 are particularly useful in wireless telecommunications applications.
The quality factor, Q, is a measure of the efficiency of a microwave system, which relates to the degree of power loss of the system. A quality factor can be defined for a whole system, a device, or for specific components or groups of components within a device or system. As used herein and in the appended claims, the Q value refers to a quality factor for a ceramic material in the form of a disc having a diameter of about 1.33 inches and a height of about 0.5 inches. Q is a dimensionless factor equal to 1/(tan δ), where δ is the loss angle. In an ideal capacitor, the phase of the alternating current will lead the phase of the voltage by 90 degrees, whereas in all real capacitors, a power loss occurs, which is manifested in a phase deviation from the ideal 90 degrees. The difference between ideal phase angle (90 degrees) and the measured phase angle in an actual capacitor is equal to δ. As δ decreases, 1/(tan δ) increases, therefore, higher values of Q represent smaller values of δ, and thus indicate higher power efficiency for a capacitor.
Experimentally, Q can be determined by the shape of the frequency resonance peak in a graph of frequency versus signal amplitude. Typically, there is a peak in transmitted signal amplitude at the resonant frequency, and the distribution of amplitude versus frequency has a finite width. By convention, the “bandwidth” is defined as the width of the frequency distribution at one-half of the maximum amplitude. The peak frequency (resonant frequency, f) divided by the bandwidth is equal to Q. Thus high Q values indicate narrow bandwidths.
The resonant frequency, f, is the peak frequency of the microwave energy that is transmitted (i.e., not blocked) by the filter. Because power losses generally increase with increasing frequency, the Q value is dependent on the resonant frequency of the filter, and the value of Q is properly reported in combination with the resonant frequency (often the frequency is listed in parentheses after Q). For convenience, a factor which is the product of Q multiplied by the resonant frequency in GHz (hereinafter “frequency*quality factor” or Qf, in units of GHz) is often utilized in place of, or in addition to reporting the Q and frequency.
For many ceramic filter materials, f will vary with the temperature of the filter. The temperature coefficient of resonant frequency, Tf, represents the change in f per degree C increase in temperature, reported in parts per million (ppm), i.e., the number of Hz by which the frequency changes when the temperature is changed one degree C, divided by f in MHz. It is particularly desirable for Tf to be as close to zero as possible; however, in practice, a Tf in the range of about −20 to about +20 ppm is acceptable.
A number of types of ceramic materials have been utilized as microwave filters in the telecommunications industry. Many microwave dielectric materials can be classified as perovskites, which have the general structure ABO3, where the weighted sum of the oxidation states of metal ions A and B is equal to +6. Various complex perovskites, such as certain tantalates (e.g., barium magnesium tantalate, Ba(Mg1/3Ta2/3)O3 and barium zinc tantalate, Ba(Zn1/3Ta2/3)O3), have been utilized in applications requiring very high Q values (i.e., >100,000). Such complex tantalates tend to have relatively low dielectric constants of less than about 30.
Another commonly used group of perovskite materials are the titanates, such as barium titanate (BaTiO3) and various mixed-metal titanates, such as calcium magnesium titanate and zirconium tin titanate, having varying ratios of calcium:magnesium and zirconium:tin, respectively, and the like. Other important classes of titanates include the barium neodymium titanates and the barium neodymium samarium titanates.
Microwave dielectric materials also can be multiphase materials such as barium neodymium titanates combined with an excess of titanium dioxide, for example. These materials can have high dielectric constants (e.g., 90 or greater), with Q values in the range of about 5,000 to 6,000, particularly when dopants such as lead oxides and bismuth oxide are utilized.
A major challenge in modern microwave dielectric ceramic filter materials research is the development of near zero Tf materials. The achievement of relatively low Tf in filter materials having a dielectric constant in the mid-dielectric constant range of about 50-70 has been a particular challenge. Filters made from materials with relatively lower K values typically are too large, whereas filters made from materials having relatively higher K values generally sacrifice performance in Wide-Band Code Division Multiple Access (WCDMA) and Personal Communications Service (PCS) digital wireless communications applications where relatively small dimension filters are particularly important.
Thus, there is an ongoing need for ceramic microwave filter materials having a relatively low Tf, in the range of about ±20 ppm, a Q of at least about 4000, and K in the range of about 50 to about 70.